glass cutter

US /ˈɡlæs ˌkʌt.ər/
UK /ˈɡlæs ˌkʌt.ər/
"glass cutter" picture
1.

a tool with a small, hard wheel or diamond tip used for scoring and breaking glass

:
He used a glass cutter to precisely cut the mirror.
Make sure the glass cutter is sharp before you start.